FIG. 1 shows the various steps of a conventional process of manufacturing a silicon radiation detector which includes the steps of:
(1) washing the silicon wafer (FIG. 2a) PA0 (2) impurity doping (FIG. 2b) PA0 (3) impurity drifting (FIG. 2c) PA0 (4) lapping (the surface opposite to the doped layer) (FIG. 2d) PA0 (5) metal deposition (to form a surface barrier) (FIG. 2e) PA0 (6) bonding (mounting) PA0 (7) depositing electrodes
First of all, a silicon wafer 1 is washed (FIG. 2a), and a diffusion layer 3, for instance consisting of a lithium doped layer is formed on one side thereof as illustrated in FIG. 2b. The impurity in the diffusion layer 3 is then drifted toward the other side of the wafer 1. However, the drift speed in the peripheral part of the wafer 1 is substantially faster than the central part thereof so that a drift layer 4 having an uneven thickness is produced as illustrated in FIG. 2c.
The unevenness in the drift speed may be attributed to an unevenness in the distribution in the impurities in the wafer. When fabricating a wafer, a cylindrical silicon crystal is grown in a furnace, and this process involves a solidification of silicon from a liquid phase to a solid phase. The central part of the silicon crystal tends to solidify later than the peripheral part thereof. Therefore, impurities such as phosphorus and crystal defects tend to be concentrated in the central part. As a result, when the silicon crystal is sliced into wafers, the central part of each wafer contains impurities and defects more in the central part than the peripheral part. Because such impurities and defects impede the drift process, the drift speed in the peripheral part of the wafer is greater than that in the central part.
Therefore, the other side which has failed to be formed into a uniform drift layer is ground or lapped so as to expose the drift layer 4 over the entire surface as illustrated in FIG. 2d. Then, a surface barrier layer 5 is formed on the lapped surface as illustrated in FIG. 2e.
In the step (3) of impurity drifting, because the drift speed in the peripheral part of the wafer is higher than that in the central part, when the impurity has reached the other side of the wafer in the peripheral part, the impurity may not have still reached the other side in the central part. Therefore, conventionally, it was necessary to carry out the step (4) of lapping the opposite surface of the wafer so that the drifted impurity may show over the entire opposite surface of the wafer.
Thus, the conventional process has the following disadvantages.
(a) The step of lapping off a substantial thickness of the substrate was required upon completion of drift.
(b) The step of depositing a metallic layer to form a surface barrier layer was required upon completion of drift. This metal deposition step requires the metal to be processed at a high temperature (of 500.degree. C. or higher). However, because the drift layer was formed by drifting a doped layer at a temperature in the order of 150.degree. C., the drift layer may be destroyed during such a high temperature deposition step.
(c) Because a surface barrier type diode is formed in a drift-type silicon radiation detector fabricated by such a conventional process, the device is vulnerable to an adverse environment (pressure changes, humidity and foreign particles), and may suffer degradation over time.
(d) The drift speed tends to be uneven because of the uneven distribution of the impurity in the silicon substrate, and this causes some difficulty in fabricating a large-diameter (three inches or larger), large-thickness (5 mm or larger) radiation detector.